Mononuclear phagocytes are prominent in host defenses against many infectious microorganisms, especially those characterized as facultative intracellular parasites. Alveolar macrophages are especially important in defending the lung against airborne organisms, and maintaining the sterility of the lower pulmonary tract. Until recently, little was known of the microbicidal mechanisms of any mammalian macrophage. In recent years, the potential importance of oxidative metabolites (superoxide, H2O2 and OH) has emerged from studies with mouse peritoneal macrophages, and we have discovered the presence of two unusual microbicidal peptides, MCP1 and MCP2, in highly purified preparations of rabbit alveolar macrophages. Our revised proposal entails completion of the molecular characterization of these peptides including determination of their amino acid sequences, precise definition of their opsonic activity and antimicrobial spectrum, and in vitro studies of their synthesis and secretion, and studies comparing MCP1 and 2 with the lysosomal cationic peptides of rabbit granulocytes. We will examine the potential interactions between MCP1 and MCP2 and other constituents of alveolar macrophages (lysozyme, etc.), and test them for synergistic effects with H2O2 and O2- producing systems analogous to those of macrophages. We will also examine alveolar macrophages for the presence of additional microbicidal components, utilizing microbial species naturally resistant to MCP1 and MCP2. In addition, by selecting resistant mutants from wild-type strains that are highly susceptible to MCP1 and MCP2, we will assess the contribution of these peptides to the microbicidal activity of intact macrophages. As these studies are completed, we will survey additional species to determine whether peptides homologous to MCP1 and MCP2 exist in the alveolar macrophages of other common laboratory animals, such as the mouse and guinea pig. Scientific disciplines and relatedness to health: our proposals are involved in the disciplines of cellular immunology and infectious disease. Given the explosive growth of bioengineering skills during the recent past, the search for natural peptide antibiotics, such a MCP1 and MCP2, especially if they prove to be widespread in their species distribution, may ultimately lead to therapeutic potentials.